1. Field of the Invention
The invention relates to a circuit arrangement for operating a LED array and comprising                a first LED driver for supplying a current to a first part of the LEDs in the LED array and equipped with                    a first switching means for adjusting the amount of current supplied to the first part of the LEDs,            a first control circuit for generating a first control signal for controlling the conductive state of the first switching means,            a first control loop for controlling the amount of light generated by the first part of the LEDs at a level represented by a first reference signal by adjusting the duty cycle of the first control signal,                        a second LED driver for supplying a current to a second part of the LEDs in the LED array and equipped with                    a second switching means for adjusting the amount of current supplied to the second part of the LEDs,            a second control circuit for generating a second control signal for controlling the conductive state of the second switching means,            a second control loop for controlling the amount of light generated by the second part of the LED at a level represented by a second reference signal by adjusting the duty cycle of the second control signal.                        
The invention also relates to a liquid crystal display unit and a back light for use in a liquid crystal display unit.
2. Description of Related Art
A circuit arrangement as mentioned in the opening paragraph is known. The known circuit arrangement is in addition to the first and second LED driver equipped with a third LED driver comprising a third control loop for controlling the amount of light generated by a third part of the LEDs. In case the known circuit arrangement is used to operate a LED array with red, green and blue LEDs, the first, second and third LED driver drive the red, green and blue LEDs respectively. White light of different colors can be generated by the known circuit arrangement by adjusting the amounts of red, green and blue light generated by the LED array. Since each of the LED drivers is equipped with its own control loop for controlling the amount of generated light, a small decrease in the efficiency of the LEDs is compensated by the control loop by installing a slightly bigger duty-cycle. In that way the color and the amount of white light are both controlled by the first, second and third control loop. However, the efficiency of LEDs, more in particular LEDs generating red light, is very strongly influenced by temperature and by aging of the LEDs. As a consequence, the duty cycle of the control signal of the switching means of the LED driver that drives the red LEDs can in practice often become equal to 100%. In case the duty cycle is encoded as a binary figure in a memory, the memory can overflow resulting in instabilities such as flashing. Furthermore, since the duty cycle cannot increase to values higher 100%, a further decrease in the efficiency of the red LEDs results in an undesired color shift of the “white” light, since the LED array is not generating enough red light.
The same problem as described hereabove can also occur in case the circuit arrangement comprises only two LED drivers, because the desired color of the light generated by the LED array can be generated by mixing only two colors instead of three.